Unless otherwise indicated herein, the description provided in this section is not itself prior art to the claims and is not admitted to be prior art by inclusion in this section.
A typical wireless network includes a number of base stations each radiating to provide coverage in which to serve wireless communication devices (WCDs) such as cell phones, tablet computers, tracking devices, embedded wireless modules, and other wirelessly equipped devices. In turn, each base station may be coupled with network infrastructure that provides connectivity with one or more transport networks, such as the public switched telephone network (PSTN) and/or the Internet for instance. With this arrangement, a WCD within coverage of the wireless network may engage in air interface communication with a base station and may thereby communicate via the base station with various remote network entities or with other WCDs served by the wireless network.
Further, a wireless network may operate in accordance with a particular air interface protocol (i.e., radio access technology), with communications from the base stations to WCDs defining a downlink or forward link and communications from the WCDs to the base stations defining an uplink or reverse link. Examples of existing air interface protocols include, without limitation, wireless wide area network (WWAN) protocols such as Orthogonal Frequency Division Multiple Access (OFDMA) (e.g., Long Term Evolution (LTE) and Wireless Interoperability for Microwave Access (WiMAX)), Code Division Multiple Access (CDMA) (e.g., 1×RTT and 1×EV-DO), and Global System for Mobile Communications (GSM), and wireless local area network (WLAN) protocols such as IEEE 802.11 (WIFI), BLUETOOTH, and others. Each protocol may define its own procedures for registration of WCDs, initiation of communications, handover between coverage areas, and/or other functions.
In practice, a base station may provide service to WCDs on carrier frequencies or “carriers.” Each carrier could be a time division duplex (TDD) carrier that defines a single frequency channel multiplexed over time between downlink and uplink use, or a frequency division duplex (FDD) carrier that defines two separate frequency channels, one for downlink communication and one for uplink communication. Each frequency channel of a carrier may then occupy a particular frequency bandwidth (e.g., 1.4 MHz, 3 MHz, 5 MHz, 10 MHz, or 20 MHz) defining a range of frequency at a particular position (e.g., defined by a center frequency) in a radio frequency band (e.g., in the 800 MHz band, the 1.9 GHz band, or the 2.5 GHz band).
For instance, in an LTE system, the air interface is divided over time into frames and sub-frames each defining two slots, and the uplink and downlink channels are each divided over their frequency bandwidth into sub-carriers that are grouped within each slot or sub-frame into physical resource blocks (PRBs). When a WCD is positioned within coverage of a base station, the WCD may register or “attach” with the base station on a particular carrier on which the base station provides service, and the base station may then serve the WCD on that carrier, scheduling particular downlink and uplink PRBs on that carrier to carry data communications to and from the WCD. Further, the base station and WCD may modulate their air interface data communications according to a modulation scheme selected based on quality of the WCD's coverage, such as with a higher-order modulation scheme when the WCD is in better coverage of the base station and with a lower-order modulation scheme when the WCD is in worse coverage.
In such an LTE system, for instance, when the base station has data to transmit to the WCD, the base station may select certain downlink PRBs to carry the data and may determine a modulation scheme for transmission on those PRBs, and the base station may then (i) transmit to the WCD a scheduling directive instructing the WCD to receive the data on the scheduled PRBs using the determined modulation scheme, and (ii) transmit the data on the indicated downlink PRBs using the determined modulation scheme. Likewise, when the base station receives from the WCD a request for the WCD to transmit data to the base station, the base station may select certain uplink PRBs to carry the data, and the base station may then (i) transmit to the WCD a scheduling directive instructing the WCD to transmit the data on the scheduled PRBs using a particular modulation scheme and (ii) receive the transmission from the WCD accordingly.
With such an arrangement, the overall bandwidth of the carriers on which a base station is configured to serve WCDs may pose an effective limit on the number of WCDs that the base station can serve, as the overall bandwidth would define only a limited number of PRBs per carrier. And the number of WCDs that the base station can serve may be further limited based on air interface conditions between the base station and each served WCD, as poor air interface conditions may necessitate assigning more PRBs to the served WCDs.
One way to help overcome this limitation is to have a base station serve multiple WCDs on the same PRBs at once, providing what is known as multi-user multiple-input multiple-output (MU-MIMO) service. With MU-MIMO service, the base station may share PRBs among a group of WCDs to increase the effective number of PRBs available to the base station by scheduling uplink or downlink communication to each WCD of the group on the same PRBs.
To facilitate this in practice, a base station configured to support MU-MIMO service typically uses multiple antennas for both transmission and reception. With multiple antennas, the base station can employ spatial multiplexing to concurrently transmit multiple downlink data streams to multiple spatially distributed WCDs on the same shared PRBs by using a different antenna for each data stream. For instance, the base station may determine appropriate beamforming patterns or directions for each antenna to mitigate interference between the downlink data streams transmitted concurrently on the shared PRBs. Likewise, the base station can employ spatial multiplexing to separate a combination of uplink data streams received from spatially distributed WCDs assigned to the same shared PRBs. For instance, although the uplink data streams are transmitted concurrently by the WCDs on the shared PRBs, each antenna may receive the uplink data streams from different directions or with different time lags due to the spatial distribution of the WCDs, and the base station may use these differences as “spatial signatures” to separate the uplink data streams.
Ideally, to maximize the number of PRBs available for scheduling to its served WCDs, a base station could provide MU-MIMO service to all the WCDs. Unfortunately, however, doing so might reduce the overall throughput of the base station due to interference caused by spatial multiplexing. By way of example, if a group of WCDs are not appropriately distributed spatially, interference between data communicated with the WCDs on shared PRBs may reduce the reliability of the air interface connections between the WCDs and the base station.
To efficiently serve WCDs, the base station may decide which WCDs the base station should group or “pair” for sharing PRBs among each other. For instance, if enabling MU-MIMO service for a group of WCDs may contribute to a throughput gain or other efficiency improvement for the base station, then the base station may commit additional signal processing overhead (e.g., for separating combined signals, etc.) associated with enabling MU-MIMO service for the group. Whereas, for instance, if enabling MU-MIMO service may contribute to deterioration and/or insignificant improvement of the base station's efficiency, then the base station may perhaps serve the group of WCDs without MU-MIMO service.
To facilitate this, the base station may keep track of each served WCD's channel state, such as the WCD's signal strength and quality, and may adjust or set the base station's MU-MIMO service accordingly. For instance, if a group of WCDs have good channel states for a particular channel, the base station may identify the group of WCDs as candidates for receiving MU-MIMO service efficiently using PRBs in that particular channel, as the good channel states may indicate low interference in that particular channel between the group of WCDs.
To do this in practice, for instance, the base station could transmit a reference signal to some or all of its served WCDs using downlink PRBs in a particular channel. Each WCD may then receive and decode the reference signal, establish one or more channel state metrics indicating signal strength and quality measurements for the received reference signal, and generate and transmit one or more associated channel state reports to the base station on uplink signaling channels and/or uplink traffic. The base station could thus receive these reports and keep a record of each WCD's latest indicated channel state, so that the base station may identify which of its served WCDs have sufficiently high signal strength and quality for receiving MU-MIMO service efficiently using PRBs in that particular channel.
Therefore, even with MU-MIMO service, the effective number of WCDs that a base station can serve may depend on how many of the served WCDs meet certain MU-MIMO efficiency criteria, such as threshold high signal strength and quality for example.